Organic EL devices are known to be highly efficient and are capable of producing a wide range of colors. Useful applications such as flat-panel displays have been contemplated. Representative of earlier organic EL devices are Gurnee et al U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, "Double Injection Electroluminescence in Anthracene," RCA Review, Vol. 30, pp.322-334, 1969; and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. Typical organic emitting materials were formed of a conjugated organic host material and a conjugated organic activating agent having condensed benzene rings. Naphthalene, anthracene, phenanthrene, pyrene, benzopyrene, chrysene, picene, carbazole, fluorene, biphenyl, terphenyls, quarterphenyls, triphenylene oxide, dihalobiphenyl, trans-stilbene, and 1,4-diphenylbutadiene were offered as examples of organic host materials. Anthracene, tetracene, and pentacene were named as examples of activating agents. The organic emitting material was present as a single layer medium having a thickness much above 1 micrometer. Thus, this organic EL medium was highly resistive and the EL device required a relatively high voltage (&gt;100 volts) to operate.
The most recent discoveries in the art of organic EL device construction have resulted in devices having the organic EL medium consisting of extremely thin layers (&lt;1.0 micrometer in combined thickness) separating the anode and cathode. Herein, the organic EL medium is defined as the organic composition between the anode and cathode electrodes. In a basic two-layer EL device structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. The extremely thin organic EL medium offers reduced resistance, permitting higher current densities for a given level of electrical bias voltage. Since light emission is directly related to current density through the organic EL medium, the thin layers coupled with increased charge injection and transport efficiencies have allowed acceptable light emission levels (e.g. brightness levels capable of being visually detected in ambient light) to be achieved with low applied voltages in ranges compatible with integrated circuit drivers, such as field effect transistors.
Further improvement in organic EL devices such as color, stability, efficiency and fabrication methods have been disclosed in U.S. Pat. Nos: 4,356,429; 4,539,507; 4,720,432; 4,885,211; 5,151,629; 5,150,006; 5,141,671; 5,073,446; 5,061,569; 5,059,862; 5,059,861; 5,047,687; 4,950,950; 4,769,292, 5,104,740; 5,227,252; 5,256,945; 5,069,975; 5,122,711; 5,366,811; 5,126,214; 5,142,343; 5,389,444; and 5,458,977.
For the production of full-color EL display panel, it is necessary to have efficient red, green and blue (RGB) EL materials with proper chromaticity and sufficient luminance efficiency. The guest-host doped system offers a ready avenue for achieving such an objective, mainly because a single host with optimized transport and luminescent properties may be used together with various guest dopants leading to EL of desirable hue.
A doped EL system based on the principle of guest-host energy transfer to effect the spectral shift from tris-(8-hydroxyquinolinato)aluminum (Alq) to the dopant molecules has been disclosed by Tang et al in commonly assigned U.S. Pat. No. 4,769,292. Alq is a suitable host for red EL emitters since its emission at 530 nm is adequate to sensitize guest EL emission in the red spectral region. The preferred dopants chosen to provide the green emission in this prior art were 3-(2'-benzothiazolyl)-7-diethylaminocoumarin, commonly known as coumarin-6 (or C-6) and 10-(2-benzothiazolyl)- 1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]q uinolizin-11-one, better known as coumarin-545T (or C-545T). The use of these green fluorescent dyes in electroluminescent devices have been described in U.S. Pat. No. 4,769,292 and Japanese Kokai No. 6-9952 (or JP 06009952-A), respectively. These molecules generally have a high photoluminescence (PL) quantum yield in dilute solution and their green electroluminescent efficiencies are high when doped at the appropriate concentration level in EL devices using Alq as host emitter. ##STR2## However, these coumarin derivatives usually have relatively low glass transition temperatures (Tg) which will impact on the thermal stability of the devices. Coumarin-6 does not have an observable Tg in DSC as it possesses a high degree of crystallinity which can ultimately impact on the thin-film morphology of the emitter layer in the device. Furthermore, in EL applications, there are always needs to modify the dopant structure to enhance the EL efficiency in the devices to save the power consumption. The higher the EL efficiency in terms of cd/A in a given device structure and drive condition, the less power it will need to produce the desired green emission with a set of the desired 1931 CIE x,y coordinates. Specific substituents, therefore, are needed to increase the Tg of the green coumarin dopants in order to increase the thermal stability of the EL devices without significantly effecting the color of the emission. Specific substituents are also needed to enhance the EL efficiency of a given green emitting EL device based on coumarin/Alq (guest/host) emitter without significantly effecting the color of the emission in terms of its CIE x,y coordinates. Accordingly, it is desirable to provide a high Tg fluorescent compound in the class of coumarins useful in EL applications which has an enhanced high EL efficiency and a desired EL emission in the green which is similar in color as C-6 or C-545T, respectively.